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Could magnetic fields have heated up the Universe while it was very young? This question was addressed with state-of-the art numerical simulations by astrophysicists at the Hamburg Observatory and the University of Manchester.

Generally, magnetic fields play an important role in galaxies and are observed in clusters of galaxies out to large cosmological scales. If magnetic fields were already present in the very early Universe shortly after the hot big bang, they would affect the evolution history of the Universe. A key effect is the energy release or heating produced by magnetic field decay and dissipation. This is especially interesting around the recombination epoch when the Universe was approximately 375,000 years old. The Universe transitioned from being an opaque particle-radiation soup to the formation of atoms whereby radiation could propagate freely producing the cosmic microwave background radiation we see today. The heating effect produced by cosmic magnetic fields after this epoch was not accurately known so far as it depended on the build up of turbulence in baryonic matter.

By performing numerical simulations of magnetohydrodynamics with the drag effect of cosmic radiation, three different phases of heating were found and investigated using magnetic fields of different strengths and spectra. In the first drag-dominated regime, prior to recombination, no real heating was found. In a newly elucidated second transition regime, around recombination, the heating rate steadily rises to a peak as baryon velocities are built up. In the third turbulent decay regime, the heating rate remains appreciable and declines slowly as turbulence is developed. Importantly, both the heating peak and the onset of turbulent decay are delayed significantly beyond recombination, by up to 500,000 years (more than the age of the Universe at recombination), for scale-invariant magnetic fields.

These results are also important for future constraints on magnetic fields from the cosmic microwave background radiation as well as the impact of magnetic fields on the formation of the first structures in the Universe and the epoch of cosmic dawn.

Animations from the simulation can be found here: https://www.hs.uni-hamburg.de/research/cosmo-mf

The published paper with details can be found here: https://doi.org/10.1093/mnras/sty1757 or https://arxiv.org/abs/1805.05315